In the conventional reduction of .alpha., .beta.-unsaturated aliphatic ketones, particularly compounds such as carvone, the reaction is complicated by competing reactions such as the concurrent reduction of the C=O bond. This difficulty is compounded if the molecule contains additional olefinic sites, such as in the case of carvone, which could also be reduced.
For example, the catalytic reduction of carvone as conventionally carried out, e.g., with hydrogen in the presence of platinum or palladium, yields multiple products resulting from the hydrogenation of all of the unsaturated sites of carvone as follows: ##STR2##
The selective reduction of the conjugated double bond, especially in compounds such as carvone, which contain additional unsaturated sites, has generally required the use of dissolving metal reductions or electrolyses. These processes are often costly and difficult to carry out on a commercial scale.
The selective catalytic reduction of unsaturated compounds with a cobaloxime is known. This reaction was reported in the Bulletin of the Chemical Society of Japan Volume 44, pages 283-285 (1971), by Yoshiaki Ohgo, Seiji Takeuchi and Juji Yoshimura, and it was stated that the reduction with a cobaloxime such as bisdimethylglyoximato(pyridine)cobalt was useful with such compounds as activated olefins, unsaturated nitrogen compounds, .alpha.-diketones and .alpha.-keto acid esters.
The reactions reported by Ohgo et al. was carried out with a molar ratio of about ten to one with regard to substrate or compound reduced to cobalt and thus was relatively inefficient in terms of moles of substrate reducible per mole of catalyst employed. This makes the process somewhat less desirable, particularly for large commercial operations, due to the cost of catalyst.
Tests were carried out with a number of different compounds or substrates many resulting in high yields of selectively reduced desired products.
For instance, the substrate ##STR3## Ph being phenyl and Et being ethyl, was selectively reduced in 7 hours reaction time to the product ##STR4## with 88% yield of the desired product.
The only example given by Ohgo et al. consisting of an olefin activated by a ketone was 4-phenylbut-3-ene-2-one, which did not react. This compound has a phenyl group on the carbon beta to the ketone which may serve to decrease the activating effect of the ketone.
Attempts by the present applicant to apply the reaction of Ohgo, et al. to the reduction of carvone resulted in selective reduction of the activated olefin giving dihydrocarvone but only in about 40% conversion.
For purposes of the present application, a cobaloxime is the complex (or its dimer) comprised of a glyoxime, cobalt and a nitrogen or phosphorous Lewis base, as illustrated by the following formula, ##STR5## wherein B is the base and R is hydrogen, or a lower alkyl or aryl hydrocarbon radical containing up to 10 carbon atoms. The complex is described in G. N. Schrauzer and R. S. Windgassen, Chem. Berichte, 99, 602 (1966) and is prepared by dissolving cobalt chloride and a glyoxime, such as dimethylglyoxime (C.sub.4 H.sub.8 O.sub.2 N.sub.2) (molar ratio 1:2) in a solvent such as methanol and adding two equivalents of a base such as sodium hydroxide (preferably in methanol) and one equivalent of the Lewis base, e.g. pyridine. A slight excess of sodium hydroxide and pyridine (1.1-1.5 times) is sometimes used to avoid possible problems from a shortage of these reagents.
To facilitate nomenclature, the term "cobaloxime" describes the bisdialkyl- or the bisdiaryl-glyoximato (base) cobalt moiety. Thus the compound wherein R is methyl and B is pyridine is named bisdimethylglyoximato(pyridine)cobalt and can be referred to as (pyridine) cobaloxime. Other representative bases are triphenylphosphine and triethylamine. These bases form the compounds (triphenylphosphine)cobaloxime and (triethylamine)cobaloxime, respectively. In the above nomenclature, it is apparent that the term (cobaloxime) is generic to the different substituted derivatives of the glyoxime (CH:NOH).sub.2 portion of the molecule. Thus the term embraces the use not only of dimethylglyoxime, but other derivatives such as diphenylglyoxime, methylphenylglyoxime and even glyoxime itself. Dimethylglyoxime is the compound most available commercially and is used by way of example in this application.
For purposes of the present application, the term "substrate" refers to the compound to be reduced or hydrogenated. The term "cycles" as employed herein refers to moles of substrate reduced per mole of catalyst employed (or alternatively per mole of dimethylglyoxime employed). "Conversion" is percent of substrate reduced.